This invention relates to hot-melt inks and, in particular, to a hot-melt ink for use in a fusible ink sheet of the type used for thermal transfer printing.
The use of thermal transfer recording has increased in recent years and various types of hot-melt inks have been proposed. These inks must change from a solid phase to a liquid phase and back to a solid phase in the short period of time during which heat is applied in order to accomplish effective thermal transfer. Waxes are known substances that are capable of undergoing these phase changes. Therefore, heat transfer inks are commonly prepared by dispersing a coloring material such as a pigment and/or a dye such as carbon black in a natural or synthetic wax primarily containing hydrocarbons. A small amount of synthetic resin or plasticizer can also be added to improve film strength, adhesiveness, flexibility and the like.
An increasing amount of research activity has recently been directed to the problem of superimposing heat transfer inks of different colors in transfer type color printers. In general, heat transfer inks have reduced overlap efficiency. Specifically, when a cyan ink is transferred onto another ink, for example a magenta ink, the density of the cyan ink is significantly lower than the density obtained when the cyan ink is transferred directly onto plain paper. A similar reduction in transfer efficiency occurs when multicolor printing using combinations of yellow, magenta, cyan and black inks is attempted. This is the primary disadvantage of transfer type color printers designed to produce prints having intermediate color tones.
A number of attempts have been made to overcome these problems including adding tackifiers to the ink layer and lowering the ink layer melting point. These attempts are effective for improving transfer efficiency when two or more inks are used but have given rise to a number of new problems as described below.
Fusible ink sheets generally include a substrate having a hot melt ink provided on one side and an electrothermal resistive layer provided on the other side. Blocking is the undesirable adhesion that occurs between the ink layer and the substrate when the transfer sheet is wound on a roll with the layers disposed on top of each other. Since the addition of a tackifier to an ink layer naturally increases tackiness, blocking becomes more likely. For example, wax sticks to the thermal head thereby lowering thermal efficiency. In addition, if the ink adheres to the resistive layer, the resistance becomes so high that transfer is no longer possible. Blocking is particularly disadvantageous in full color printing as it becomes difficult to express a gradation of shades due to insufficient optical density of the inks or an inability to control optical density.
When low melting point inks are used, a first transferred ink is melted when a second ink of a different color is transferred onto the first ink. As a result, the second ink is mixed with the first ink in a molten state to achieve improved transfer efficiency. However, low melting point inks also lower the temperature at which blocking occurs.
It is, therefore, desirable to provide a hot-melt ink that can be transferred onto another ink as efficiently as it can be transferred onto paper and which has a high degree of blocking resistance.